Thermocouple input temperature sensing circuitry

ABSTRACT

Voltage sensing circuitry for use with a thermocouple having a voltage output which is related to the temperature thereof. The thermocouple output voltage is compared with a reference voltage and the difference is amplified. The reference voltage varies with ambient temperature in order to null out the effects of temperature change on the cold junction of the thermocouple circuit, the cold junction being disposed within the same region as that in which the temperature indicating circuitry is disposed. The differential voltage between the thermocouple output voltage and the reference voltage is amplified by a single-ended amplifier so that the output of the thermocouple has low impedance to a conductor which is common to a power supply, to the reference voltage, and to output receiver means. Thus, the circuitry has high common mode rejection, i.e., there is insignificant effect upon the output voltage of the circuitry if the thermocouple comes into contact with an external voltage.

United States Patent Arnett et al.

[ 51 Mar. 21, 1972 [54] THERMOCOUPLE INPUT TEMPERATURE SENSING,CIRCUITRY [72] Inventors: Charles J. Arnett, Union; James M.

Hoover, New Carlisle, both of Ohio [73] Assignee: Instrulab, lnc.,Dayton, Ohio [22] Filed: Feb. 25, 1970 [21] App]. No.: 13,998

Primary Examiner-S. Clement Swisher Assistant ExaminerFrederick ShoonAtt0rney-Jacox & Meckstroth [57] ABSTRACT Voltage sensing circuitry foruse with a thermocouple having a voltage output which is related to thetemperature thereof. The thermocouple output voltage is compared with areference voltage and the difference is amplified. The reference voltagevaries with ambient temperature in order to null out the effects oftemperature change on the cold junction of the thermocouple circuit, thecold junction being disposed within the same region as that in which thetemperature indicating circuitry is disposed. The differential voltagebetween the thermocouple output voltage and the reference voltage isamplified by a single-ended amplifier so that the output of thethermocouple has low impedance to a conductor which is common to a powersupply, to the reference voltage, and to output receiver means. Thus,the circuitry has high common mode rejection, i.e., there isinsignificant effect upon the output voltage of the circuitry if thethermocouple comes into contact with an external voltage.

5 Claims, 2 Drawing Figures REFERENCE JUNCTION ILINEARIZER l ,60 23/EIIHIIH VOLTAGE SENSING 20 DEVICE THERMOCOUPLE INPUT TEMPERATURE SENSINGCIRCUITRY BACKGROUND OF THE INVENTION Various thermocouple typetemperature sensing devices have been created. However, in order toobtain high impedance circuitry to the thermocouple, the common practiceis to convert the direct current thermocouple signal to alternatingcurrent by means of a chopper or the like. Then the resultantalternating current signal is passed through a transformer and amplifiedby an alternating current amplifier. Then the output of the alternatingcurrent amplifier is converted back to direct current. This directcurrent signal is then linearized and read. Such circuitry thus requirescostly and bulky components, and is therefore objectionable.Furthermore, while this conventional type of circuitry does produce highimpedance to the thermocouple, it also produces low common moderejection. An isolation transformer is used to overcome this low commonmode rejection condition. In addition to these objectionable features,known thermocouple responsive circuitry requires careful relativepositioning of the components of the circuitry.

It is an object of this invention to provide thermocouple type sensingcircuitry which has high input impedance characteristics and which alsohas high common mode rejection, without the use of a transformer, or achopper, or an AC to DC converter.

It is another object of this invention to provide such circuitry whichdoes not require careful relative positioning of components and whichcan be constructed in small physical size and at relatively low costs.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 of the drawing is a schematicdiagram of thermocouple input temperature sensing circuitry of thisinvention.

FIG. 2 is a schematic diagram of a portion of circuitry which may beincluded as a part of the circuitry of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The circuitry of this inventionis adapted to be joined to a thermocouple which is shown as beinglocated in a position or region 11, the temperature of which is sensedby the circuitry of this invention. The thermocouple 10 is, preferably,a junction type of device which has an output voltage which is relatedto the temperature thereof.

The thermocouple 10 has cold junction terminals 12 and 14 which areexterior of the region 11 within which the thermocouple I0 is located.The terminal 12 is joined to a reference junction unit 16, which is anysuitable unit which is capable of producing a voltage which is directlyrelated to the temperature thereof.

The reference junction unit 16 is also connected by a conductor 13 tothe positive portion of a power supply or source of electrical energy20. A conductor 25 joins the reference junction unit 16 to a midportionof the power supply 20, and a conductor 27joins the reference junctionunit 16 to a negative portion of the power supply 20. Herein the powersupply or source of electrical energy 20 is shown as consisting of abattery. However, the power supply 20 may be any other suitable sourceof direct current electrical energy, such a rectified output of analternating current source of electrical energy, or the like.

Preferably, but not necessarily, the reference junction unit 16 includescircuitry shown in FIG. 2 which comprises an impedance element which isjoined to the terminal 12 and which is also joined to the power supplyor source of electrical energy 20. An impedance element 17 is alsoconnected to the terminal 12 and to a junction between an impedanceelement 19 and an impedance element 21. The impedance elements 19 and 21are connected across a portion of the power supply 20. The impedancevalues of the elements 17 and 19 are relatively low.

As shown in FIG. 1, the power or source of electrical energy 20 is alsojoined to a high impedance solid state power amplifier, herein shown asa metal oxide semiconductor field-effect transistor 24, which, forexample, may be a transistor referred to by RCA as Number 3N138. Thesource of electrical energy 20 is connected by a conductor 23 to a drainelectrode 22 of the field-effect transistor 24. The metal oxidesemiconductor field-effect transistor 24 has a source electrode 26 whichis connected to an impedance element or resistor 30 and which is alsoconnected to a linearizer 32. The linearizer 32 comprises any suitablelinearizing circuit which senses the nonlinearity of the amplifiedthermocouple signal voltage at the source electrode 26 and in apredetermined program converts the nonlinear signal into a linear signalin which the output of the linearizer 32 is a linear function oftemperature. A suitable linearizing circuit has been found to be a solidstate function generator in which the amplified signal is applied to aresistor divider network which at predetermined levels of amplifiedsignal, the ratio of the resistor divider network is changed via activeand passive components. This ratio is changed so that while the input isa nonlinear function of temperature, the output is a linear function oftemperature. The source electrode 26 is also joined to an impedanceelement or resistor 3l, which is connected by a conductor 33 to thenegative portion of the power supply 20.

A conductor 18 is connected to the conductor 25. The resistor 30 is alsojoined to an impedance element or resistor 34 which is connected to theconductor 18. The resistor 34 is also connected by a conductor 35 to asignal input terminal 36 of a high gain voltage amplifier 38. Theamplifier 38 is preferably one which has very low input offset voltagedrift, i.e., a drift which may result from temperature and/or time. Theamplifier 38 may, for example, be an amplifier referred to by AnalogDevices Company, Cambridge, Massachusetts, as Model Number 232 1.However, other amplifiers may also be satisfactory. The high gainvoltage amplifier 38 also has a signal input terminal 40, which isconnected by a conductor 41 to an intermediate tap 42 of a source ofelectrical energy or power supply 46. The power supply or source ofelectrical energy 46 may be any suitable source of direct currentelectrical energy, such as rectified alternating current energy, or abattery, as shown, or the like. The power supply 46 is also joined topower supply input terminals 50 and 52 of the high gain amplifier 38.

The amplifier 38 has an output terminal 54 which is connected to a gateelectrode 56 of the transistor 24.

The linearizer 32 is connected to any suitable voltage sensing device60, which as an indicator or recording meter or instrument, whichindicates and/or records and/or controls temperature. The device 60 may,if desired, be a digital indicating meter. The voltage sensing device 60and the linearizer 32 are also connected to the conductor 18.

OPERATION The thermocouple 10 is disposed within a position or region11, the temperature of which is sensed and/or controlled by circuitry ofthis invention. The thermocouple 10 produces a voltage which changeswith the temperature of the region 11. All of the other elements shownin the figures of the drawing are in a region outside the region 11 andhave a temperature normally different from that of the region 11.

The reference junction unit 16, which is in series with the thermocouple10, provides a voltage which has a direct relationship to thetemperature of the elements other than the thermocouple 10. For example,the voltage output of the reference unction unit 16 may be based uponzero degrees centigrade or Fahrenheit so that no voltage output appearsbetween terminal 14 and the conductor 18 when the region 11 is at zerodegrees. When the circuitry of FIG. 2 is employed as a part of thereference junction unit 16, the impedance elements I5, 17, 19 and 21 areadjusted so that when the region 11 is at zero degrees, substantially novoltage exists between the terminal 14 and the conductor 18, regardlessof the ambient temperature within which the circuitry is disposed. Theimpedance of the impedance element 17 is directly related to thetemperature thereof. Thus, a change in ambient temperature produces avoltage change between the terminal 12 and the conductor 18. This changenulls out any changes in voltage at the terminals 14 and 12 caused bychange in ambient temperature. Thus, regardless of the temperature ofthe elements outside the region 11, the voltage between the conductorsl8 and 41 is continuouslyproportional to the temperature within theregion 1 1, as sensed by the thermocouple 10.

The conductor 41 provides an input to the amplifier 38 at the terminal'40, and the conductor 35 provides an input to the amplifier 38 at theterminal 36. For reasons discussed below, the voltage applied across thesignal input terminals 36 and 40 of the amplifier 38 is normallymaintained at a substantially given predetermined value, regardless ofthe temperature sensed by the thermocouple 10.

When, for example, an increase in the temperature sensed by thethermocouple 10 occurs, there is a momentary rise or a tendency to risein the voltage applied across the signal input terminals 36 and 40 ofthe amplifier 38. Thus, the output voltage of the amplifier 38, which isapplied to the gate element 56 of the transistor 24 increases. The metaloxide semiconductor field-effect transistor 24 herein is a potentialcontrol device and is used as a source follower and serves as aninterface power amplifier. As stated above, the amplifier 38 is ahighgain amplifier, which for example, may have a gain of 10. Therefore,a change of a few microvolts in the potential applied across the signalinput terminals 40 and 36 of the amplifier 38 results in a very largechange in the potential at the output terminal 54 which is applied tothe gate electrode 56 of the transistor 24. The potential at the sourceelectrode 26 is controlled by the potential applied to the gateelectrode 56 of the transistor 24. Thus, with a rise in the potential ofthe gate electrode 56, there is a rise in the voltage at the sourceelectrode 26 with respect to the conductor 18. Thus, the potentialacross the resistors 34 and 30 increases in a direct ratio to theincrease in voltage from the source electrode 26 to the conductor 18.

In order that the potential applied across the input terminals 36 and 40of the amplifier 38 constantly remains at substantially the samevoltage, the voltage across the resistor 34 must be maintained at agiven value with respect to the voltage from the terminal 14 to theconductor 18. For this reason, the rise in voltage across the resistor34, which was initiated by a rise in output voltage of the thermocouple10, causes the potential across the input terminals 40 and 36 to belowered by a given value to maintain the voltage between the inputterminals 36 and 40 at substantially a constant value.

The power supply provides current which flows through the drainelectrode 22 and through the source electrode 26 of the transistor 24.Due to the fact that the current through the transistor 24 is suppliedby the power supply 20, an insignificant amount of current flows fromthe power supply 46 through the thermocouple 10 to the conductor 18.Thus, the circuitry has very high impedance to the terminals 12 and 14of the thermocouple 10.

As discussed above, the voltage across the resistor 34 is equal to thevoltage between the conductor 41 and the conductor 18. This voltage isalso proportional to the voltage from the source electrode 26 to theconductor 18. Thus, the voltage across the linearizer 32 is proportionalto the voltage between the terminal 14 and the conductor 18. Thisproportion is a high ratio. The voltage between the terminal 14 and theconductor 18 is directly proportional to the voltage across theterminals 14 and 12. The voltage across the terminals 14 and 12 isdirectly related to the temperature of region 11. Thus, the voltageacross the linearizer 32 is directly related to the temperature ofregion 11.

The linearizer 32 is used to compensate for the nonlinear voltage outputof the thermocouple 10. The linearizer 32 provides an output voltagewhich is directly proportional to the trode 26 of the transistor 24,provides a negative bias to the source electrode 26. Thus, when thevoltage between the terminal l4 and the conductor 18 becomes negative,the source electrode 26 becomes negative. Such conditions occur if thetemperature of the region 11 becomes lower than zero degrees. Thus, thecircuitry of this invention is capable of sensing temperatures which areeither above or below zero degrees, or which are above or below apredetermined value.

In some types of uses of a thermocouple for sensing temperature, thethermocouple, either inadvertently or by reason of the nature of thematerial in engagement therewith, comes into contact with a circuithaving a voltage other than ground potential. This is known as a commonmode voltage. Such a condition causes a current to flow through thethermocouple and through the sensing circuitry and to ground. Due to thefact that the impedance of the resistance elements 19 and 17 in thereference junction unit 16 are of low value, the voltage created by anyleakage currents, caused by the common mode voltage, is insignificant.Thus, the circuitry of this invention has high common mode rejection.

Thus, it is understood that circuitry of this invention overcomesobjections in conventional temperature sensing circuitry. Also, thecircuitry of this invention can be constructed to have a small physicalsize.

Although the preferred embodiment of the device has been described, itwill be understood that within the purview of this invention variouschanges may be made in the form, details, proportion and arrangement ofparts, the combination thereof and mode of operation, which generallystated consist in a device capable of carrying out the objects setforth, as disclosed and defined in the appended claims.

I claim:

1. Temperature sensing apparatus comprising:

a thermocouple,

a reference voltage unit connected to the thermocouple in seriesrelationship therewith,

a voltage amplifier having a signal input portion and an output portion,

means connecting the thermocouple to the signal input portion of thevoltage amplifier,

a first source of electrical energy, the first source of electricalenergy being connected to the amplifier for operation thereof,

means joining the signal input portion of the amplifier to the firstsource of electrical energy,

common conductor means,

means joining the reference voltage unit to the common conductor means,

a first impedance element,

means connecting the first impedance element to the common conductormeans,

means joining the first impedance element to the signal input portion ofthe amplifier,

a metal-oxide semiconductor field-effect transistor having a gateelectrode, a drain electrode, and a source electrode,

means joining the output portion of the amplifier to the gate electrodeof the transistor,

a second source of electrical energy,

means joining the second source of electrical energy to the drainelectrode of the transistor and to the common conductor means,

a second impedance element,

means joining the second impedance element to the source electrode ofthe transistor and to the input portion of the amplifier and to thefirst impedance element,

output receiver means,

means connecting the output receiver means to the source electrode ofthe transistor and to the common conductor means,

unit includes a first resistor and a second resistor connected to thethermocouple, the first resistor also being connected to the secondsource of electrical energy, a third resistor and a fourth resistorconnected in series across a portion of the second source of electricalenergy, the second resistor being joined to the junction between thethird resistor and the fourth resistor, the resistance of the secondresistor being directly related to the temperature thereof.

the impedance elements and the thermocouple and the reference junctionunit cooperatively applying a substantially constant voltage to thesignal input portion of the amplifier, regardless of the output voltageof the thermocouple and of the reference voltage unit, the voltage of 5the output portion of the amplifier thus being applied to the outputreceiver means in nonlinear relationship to the temperature of thethermocouple.

2. The apparatus of claim 1 in which the reference voltage 3.Temperature sensing circuitry for use with a thermocouple which isdisposed within a region, for sensing thereof, comprising:

a low impedance reference voltage unit for connection to a thermocoupleand which produces a voltage which is directly related to thetemperature thereof, the reference voltage unit being disposed in aregion other than the region within which the thermocouple is disposed,the reference voltage unit sensing the ambient temperature of the regionwithin which it is disposed,

amplifier means, the amplifier means having a high impedance solid statepower output portion, the amplifier means having a signal input portion.

voltage divider means joining the output portion of the amplifier meansand the reference voltage unit and the thermocouple to the signal inputportion of the amplifier means,

power supply means,

means joining the power supply means to the output portion and to theinput portion of the amplifier means so that the voltage applied to theinput portion of the amplifier means is related to the voltage at theoutput portion of the amplifier means,

connector means for joining the power supply means and the outputportion of the amplifier means to output receiver means,

the signal input portion of the amplifier means including a high gainvoltage amplifier portion, the high impedance solid state power outputportion including a metal oxide semiconductor field-effect transistorhaving a gate electrode, the high gain voltage amplifier portion beingjoined to the gate electrode of the metal oxide semiconductorfield-effect transistor.

4. The circuitry of claim 3 in which the metal oxide semiconductorfield-effect transistor also has a source electrode and a drainelectrode, the voltage divider means being joined to the sourceelectrode, the power supply means being joined to the drain electrode.

5. The circuitry of claim 4 in which the connector means include meansjoined to the source electrode and to the voltage divider means.

1. Temperature sensing apparatus comprising: a thermocouple, a referencevoltage unit connected to the thermocouple in series relationshiptherewith, a voltage amplifier having a signal input portion and anoutput portion, means connecting the thermocouple to the signal inputportion of the voltage amplifier, a first source of electrical energy,the first source of electrical energy being connected to the amplifierfor operation thereof, means joining the signal input portion of theamplifier to the first source of electrical energy, common conductormeans, means joining the reference voltage unit to the common conductormeans, a first impedance element, means connecting the first impedanceelement to the common conductor means, means joining the first impedanceelement to the signal input portion of the amplifier, a metal-oxidesemiconductor field-effect transistor having a gate electrode, a drainelectrode, and a source electrode, means joining the output portion ofthe amplifier to the gate electrode of the transistor, a second sourceof electrical energy, means joining the second source of electricalenergy to the drain electrode of the transistor and to the commonconductor means, a second impedance element, means joining the secondimpedance element to the source electrode of the transistor and to theinput portion of the amplifier and to the first impedance element,output receiver means, means connecting the output receiver means to thesource electrode of the transistor and to the common conductor means,the impedance elements and the thermocouple and the reference junctionunit cooperatively applying a substantially constant voltage to thesignal input portion of the amplifier, regardless of the output voltageof the thermocouple and of the reference voltage unit, the voltage ofthe output portion of the amplifier thus being applied to the outputreceiver means in nonlinear relationship to the temperature of thethermocouple.
 2. The apparatus of claim 1 in which the reference voltageunit includes a first resistor and a second resistor connected to thethermocouple, the first resistor also being connected to the secondsource of electrical energy, a third resistor and a fourth resistorconnected in series across a portion of the second source of electricalenergy, the second resistor being joined to the junction between thethird resistor and the fourth resistor, the resistance of the secondresistor being directly related to the temperature thereof. 3.Temperature sensing circuitry for use with a thermocouple which isdisposed within a region, for sensing thereof, comprising: a lowimpedance reference voltage unit for connection to a thermocouple andwhich produces a voltage which is directly related to the temperaturethereof, the reference voltage unit being disposed in a region otherthan the region within which the thermocouple is disposed, the referencevoltage unit sensing the ambient temperature of the region within whichit is disposed, amplifier means, the amplifier means having a highimpedance solid state power output portion, the amplifier means having asignal input portion. voltage divider means joining the output portionof the amplifier means and the reference voltage unit and thethermocouple to the signal input portion of the amplifier means, powersupply means, means joining the power supply means to the output portionand to the input portion of the amplifier means so that the voltageapplied to the input portion of the amplifier means is related to thevoltage at the output portion of the amplifier means, connector meansfor joining the power supply means and the output portion of theamplifier means to output receiver means, the signal input portion ofthe amplifier means including a high gain voltage amplifier portion, thehigh impedance solid state poweR output portion including a metal oxidesemiconductor field-effect transistor having a gate electrode, the highgain voltage amplifier portion being joined to the gate electrode of themetal oxide semiconductor field-effect transistor.
 4. The circuitry ofclaim 3 in which the metal oxide semiconductor field-effect transistoralso has a source electrode and a drain electrode, the voltage dividermeans being joined to the source electrode, the power supply means beingjoined to the drain electrode.
 5. The circuitry of claim 4 in which theconnector means include means joined to the source electrode and to thevoltage divider means.